Current control for microprocessor motor drive

Abstract

A motor control for controlling current in an induction motor. A microcomputer generates three inputs to control an inverter circuit that energizes the three phase windings of the motor. A speed control modulates the three inputs to adjust motor speed and voltage. A current sensing circuit also modulates the three inputs to reduce motor current using a technique that over rides the speed control. By monitoring the current sensing circuit the microcomputer adjusts the speed control to force motor current to a level near a set threshold value.

I claim: 
     
       1. A motor control for controlling energization of a multiple phase alternating current motor having multiple motor windings comprising: (a) a programmable controller for providing a shaped, time varying digital energization signal for each phase wherein the state of the time varying digital energization signal corresponds to an energization state of a motor winding corresponding to that phase;   (b) a current control circuit for monitoring motor current supplied to said motor, comparing said motor current with a threshold, and generating a modulation output to reduce the frequency and voltage of energization signals coupled to said motor when the motor current exceeds the threshold; and   (c) logic circuitry coupled to the programmable controller and the current control circuit for receiving both the time varying digital energization signals from the programmable controller and the modulation output of said current control circuit and for producing a resultant motor control output for controlling energization of the motor windings, said resultant motor control output reducing motor energization voltage across the motor windings to zero and preventing change in motor energization signals for a duration corresponding to the degree to which the motor current exceeds the threshold.   
     
     
       2. The motor control of claim 1 wherein the current control circuit comprises a sensor for monitoring the current in a D.C. bus coupled to said motor through an inverter, means for converting current in the sensor to an analog output signal, and a comparator for comparing the analog output signal with a reference signal corresponding to the threshold, said comparator having an output coupled to the logic circuitry to modulate the motor frequency and voltage applied to said motor for a duration corresponding to a time period the analog output exceeds said reference signal. 
     
     
       3. The motor control of claim 2 wherein the means for converting comprises motor current estimating means for estimating motor current when said analog output signal exceeds said reference signal and the current in the bus fails to zero. 
     
     
       4. Apparatus of claim 1 wherein the motor is a three phase motor having three motor windings, the programmable controller generates three, phase shifted, time varying digital signals to approximate sinusoidal motor winding energization wherein at each moment of motor control one of said three time varying signals is of a first state and two others of said time varying signals are of a second state and further wherein the logic circuitry changes the one time vary signal having the first state to said second state upon receipt of a modulation output from said current control circuit. 
     
     
       5. The motor control of claim 1 wherein the time varying digital signal from the programmable controller is generated in response to a clock signal source external to the programmable controller and wherein the logic circuitry includes a gate to stop clocking of the programmable controller when the motor current exceeds the threshold to prevent the change in the state of the digital energization signals. 
     
     
       6. A motor control for energizing a multiple phase motor comprising: (a) an inverter for coupling a direct current signal from a D.C. bus across multiple phase windings of said multiple phase motor, said inverter including a number of inverter switches to control an energization state of said multiple phase windings;   (b) a control unit for creating as many digital signal outputs as there are phase windings of said motor, said control unit including a programmable controller to produce a shaped time varying digital signal at each digital signal output wherein the state of the digital signal corresponds to an energization state of a corresponding motor winding; said control unit also including means for generating an adjustable repetitive digital signal to modulate the motor frequency and voltage applied to the motor windings by the inverter;   (c) logic circuitry coupled to both the time varying digital signals and the repetitive digital signal to produce resultant motor winding control outputs for controlling the opening and closing of said inverter switches; and   (d) a current control circuit for monitoring motor current through said D.C. Bus, comparing said current with a threshold current, and generating an output coupled to said logic circuitry to duplicate the modulation effects of one state of said repetitive digital signal to reduce the motor speed and voltage of the energization signals coupled through said inverter, causing said D.C. bus current to be limited to approximately the threshold current.   
     
     
       7. The motor control of claim 6 wherein the current control circuit comprises a current sensor for monitoring the current in a direct current bus coupled to said multiple motor windings through said inverter, a differential amplifier to convert current in the shunt resistor to an analog output signal, and a comparator for comparing the analog signal with a reference signal corresponding to the threshold current, said comparator having an output coupled to the logic circuitry to modulate the motor speed and voltage applied to said windings if the analog output from said differential amplifier exceeds said reference signal. 
     
     
       8. The motor control of claim 7 including a nonlinear filter to shape the D.C. bus current signal from the current sensor so that the current signal is more representative of motor current during intervals when D.C. Bus current in zero. 
     
     
       9. Apparatus of claim 6 wherein the motor is a three phase motor having three motor windings, the control unit generates three, phase shifted, time varying signals to approximate sinusoidal motor winding energization wherein at each moment of motor control one of said three time varying signals is of a first state and two others of said time varying signals are of a second state and further wherein the logic circuitry changes the one time varying signal to said second state upon receipt of a modulation signal from either said control unit or said current control circuit. 
     
     
       10. A method of operating a multiphase motor comprising the steps of: (a) programming a microprocessor controller to generate multiple controller outputs, one controller output for each phase of said multiphase motor and coordinating the relative timing of the phases of said controller outputs with respect to each other;   (b) generating a controller duty cycle frequency and voltage control signal wherein the duty cycle of said frequency and voltage control signal is adjusted to adjust the speed of the induction motor;   (c) evaluating the state of said multiple microprocessor outputs and said frequency and voltage control signal and altering said controller outputs to make the controller outputs the same state upon the occurrence of a frequency and voltage control signal of a particular state;   (d) controlling motor winding energization provided by a bus through an inverter circuit to energize motor windings of the multiphase motor in synchronization with the controller outputs as altered by the frequency and voltage control signal; and   (e) monitoring the current supplied to the multiphase induction motor through the bus and overriding the evaluating step to change the outputs to the same state if an overcurrent condition is sensed, said overriding step resulting in a reduction in current in the multi-phase motor until the current falls below a threshold.   
     
     
       11. The method of claim 10 wherein the step of generating multiple controller outputs is temporarily halted when the frequency and voltage control signal is of the particular state. 
     
     
       12. The method of claim 11 wherein the energization of the motor windings is accomplished by coupling the windings through an inverter circuit to a bus and wherein the monitoring of current is performed by monitoring current passing through the bus to the inverter circuit. 
     
     
       13. A motor control for controlling energization of multiple motor windings of a multiple phase alternating current motor comprising: (a) programmable controller means for providing a shaped, time varying digital energization signal for each phase wherein the state of the time varying digital energization signal corresponds to an energization state of a motor winding, said programmable controller means including an external clock input for providing clock pulses used by the programmable controller means in generating the time varying digital energization signals;   (b) current sensing means for monitoring motor current supplied to said motor, comparing said motor current with a threshold current, and generating a modulation output signal to reduce the frequency and voltage of energization signals coupled to said motor when an instantaneous sensed motor current exceeds the threshold current; and   (c) logic means coupled to the programmable controller means for receipt of the time varying digital energization signals from the programmable controller means and coupled to the modulation output of said current sensing means for receipt of the modulation output signal; said logic means including means for reducing motor energization voltage across the motor windings to zero by adjusting the state of the time varying digital energization signal for at least one phase of the motor and by temporarily disabling transmission of clock signals to the programmable controller means to prevent changes in a state of the digital energization signals output by the programmable controller means.   
     
     
       14. The motor control of claim 13 wherein the programmable controller means includes means for generating a pulsed waveform whose duty cycle is adjusted to change the speed of the motor and wherein the logic means includes means for combining the modulation output signal of the current sensing means and the pulsed waveform from the programmable controller means to adjust motor operation in response to either a sensed current condition or from the duty cycle set by the programmable controller.